Use of storage batteries for storage of commercial power has been examined along with progress of the battery techniques accompanied by widespread use of notebook computers and portable information apparatuses in these days. For example, Patent Document 1 discloses a configuration of an electric power storage system for commercial power using a battery module in which a plurality of compact storage batteries are connected in parallel and in series.
Recently, as application of these electric power storage systems for commercial power, smoothing of electric power generated by wind power generation and solar power generation has received attention. Moreover, it has been examined that the electric power is charged during the night when an electricity price is low, and discharged during the daytime when the electricity price is high, thereby to reduce an electricity cost for a user. Patent Document 2 discloses a configuration of an electric power storage system for commercial power in which electric power is stored during the nighttime price hours, and discharged during the daytime price hours.
In the electric power storage system using the storage battery, management of a depth of discharge is essential. This is because the storage battery is deteriorated due to overdischarge. Patent Document 2 above discloses a configuration in which a remaining capacity is estimated from the history of charge and discharge, and discharge is performed at 80% to 95% of the remaining capacity as an upper limit. Moreover, in the case of the lithium ion storage battery, it is known that by making the depth of discharge shallow, the life of the battery (lifetime capacity) is prolonged. Patent Document 3 discloses a configuration in which nighttime electricity is stored, and discharged during the daytime as in Patent Document 2, but a battery is usually used at a shallow depth of discharge. Moreover, as a related art in consideration of the cycle life of the storage battery, Patent Document 4 is disclosed.
Moreover, recently, along with liberalization of the electricity market, real time pricing in which the price of electricity dynamically changes is started to be introduced. Patent Document 5 discloses a configuration of an electric power storage system ready for not only the case where the price of electricity is fixed depending on the hours such as the nighttime and the daytime, but also the case where the price of electricity dynamically changes. In the configuration in Patent Document 5, charge is performed at a time of the day in which the electricity price is lowest, and discharge is performed at a time of the day in which the electricity price is highest even in the case of the dynamic price of electricity.